1. Field of the Invention
The present invention relates to a liquid crystal device, and more particularly to a liquid crystal display device having a color filter.
2. Description of the Related Art
With the development of various portable electronic devices such as mobile phones, PDAs, notebook computers, or the like, the demand for light, thin, and small Flat Panel Display (FPD) devices is increasing. Research is active in the area of flat panel display devices, such as an LCD (liquid crystal display), a PDP (Plasma Display Panel), an FED (Field Emission Display), a VFD (Vacuum Fluorescent Display), or the like. Currently, interest is in liquid crystal display (LCD) devices amenable to simple mass-production, using a simple driving system, and providing high picture quality.
The LCD devices have various display modes according to the arrangements of liquid crystal molecules. However, among the various display modes, Twisted Nematic (TN) mode liquid crystal display device has widely been used because of its high contrast ratio, rapid response, and low driving voltage. The TN mode liquid crystal display device includes an array substrate on which pixel regions are arranged, a color filter substrate facing the array substrate, and liquid crystals formed between the array substrate and the color filter substrate. In addition, polarizing plates are formed at outer surfaces of the array substrate and the color filter substrate such that polarized light may reach the liquid crystals. In addition, the liquid crystals are twisted in a spiral shape and arranged between the array substrate and the color filter substrate.
In a related art liquid crystal display device, gate lines and data lines are arranged on the array substrate. Thin film transistors (TFTs) are arranged on the array substrate. Color filter layers for displaying color images are formed on a color filter substrate facing the array substrate. Pixels are arranged on the TFT array substrate and the color filter substrate to form a matrix of pixels that are precisely aligned to each other. If the pixel regions formed on the TFT array substrate and the color filter substrate are imprecisely aligned, a light generated from a backlight may leak. Thus, the alignment of the TFT array substrate and the color filter substrate in the related art liquid crystal device is very important.
To address the misalignment problem, and to focus on a specific substrate for fabricating a liquid crystal display device, a liquid crystal display device having a color filter on TFT (COT) has been provided. In the COT liquid crystal device, the color filter layer is formed on the TFT array substrate. In the liquid crystal display device having the COT structure, since the color filter layers are formed on the TFT array substrate, potentially complicated processes for forming the color filter may be performed when the TFT array substrate is fabricated. The aperture ratio of the display device may be improved by forming the color filter layers on the TFT array substrate. Also, it is easier to arrange the TFT array substrate and an upper substrate when the TFT array substrate includes the color filter layers.
FIG. 1A is a plan view illustrating a pixel region of a related art liquid crystal display device having a COT structure. Referring to FIG. 1, a plurality of gate lines 102a, and a plurality of data lines 107 perpendicularly crossing the gate lines 102a, define pixel regions. In addition, a thin film transistor 130 is formed at one side of the pixel region. The thin film transistor 130 is connected to the gate line 102a and the data line 107.
A black matrix 110 is formed at an upper portion of the gate line 102a and the data line 107. The block matrix 110 blocks unnecessary light from being transmitted from under lower portions of the gate lines and the data lines. In addition, a color filter layer, including a red, green and blue sub-color filter layers (not shown), is formed in the pixel region. The color filter layer provides color display capability for the liquid crystal display device. A pixel electrode 109 is also formed in the pixel region. The pixel electrode 109 applies an electric field to liquid crystals in the pixel region.
Since the color filter layer is formed on the array substrate on which a TFT is formed, a liquid crystal display device having such a structure is referred to as a color filter on array (COA) or a color filter on TFT (COT).
FIG. 1B is a cross-sectional view along line A-A' of the related art liquid crystal display device having the COT structure of FIG. 1A. Referring to FIG. 1B, the gate line (not shown) and a gate electrode 102 diverging from the gate line are formed on a transparent substrate 101, and a gate insulating layer 103 is formed on the gate electrode 102. An active layer 104 of the thin film transistor is formed on the gate insulating layer 103, and source and drain electrodes 106a and 106b are connected to the active layer 104. An ohmic layer 105 is interposed between the source and drain electrodes 106a and 106b. The data line 107 is formed on the gate insulating layer 103. The data line 107 is connected to the source electrode 106a and concurrently formed with the source electrode 106a. A pixel electrode 109 connected to the drain electrode 106b is formed at the pixel region to thereby apply an electric field to a liquid crystal layer 120. The source and drain electrodes 106a and 106b and the data line 107 are insulated by an interlayer passivation layer 108. The black matrix 110 and a color filter layer 111 are separately formed on the passivation layer 108.
A color filer layer, including red, green and blue sub-color filter layers, is formed in each pixel region. The black matrix 110 is formed at a reverse tilt domain region, such as the gate line, the data line and a region where the TFT is formed, thereby preventing light leakage. An upper substrate is located facing the TFT array substrate. The upper substrate includes a transparent substrate 150 and the common electrode 151 formed on the substrate 150. In addition, alignment layers 112 and 152 for aligning the liquid crystals 120 may be further formed on the TFT array substrate. A liquid crystal material 120 is filled between the upper substrate 150 and the TFT array substrate 101.
When fabricating a liquid crystal display device having the COT structure, processes for forming the black matrix 110 and the color filter layer 111 are performed after forming the pixel electrode 109 in each pixel. Since the black matrix 110 and the color filter layer 111 are formed of photosensitive organic layers, photo-mask processes should be performed when forming the black matrix 110 and the color filter layer 111, respectively. The photo-mask processes increase the fabrication time of a liquid crystal display device. Moreover, a photo-mask is so expensive that fabrication costs of liquid crystal displays increase significantly.
The related art, a color filter layer is commonly formed by a pigment dispersing method in which red, green and blue sub-color filters are formed by respective photo-mask processes. The color filter layer is formed by using the pigment dispersing method. The black matrix is separately formed. Accordingly, a total of four photo-mask processes are required. There is a strong felt need in the art to reduce the number of masks.